adjusting tonicity

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Buffered and Isotonic Solutions

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Contents

• The Buffer Equation• Buffer Capacity• Buffers in pharmaceutical and Biologic Systems

• Buffered Isotonic Solutions• Methods of Adjusting Tonicity and pH

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Introduction

Buffered Solutions ?

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Buffered Solutions

0.1N HCl 1ml

pH4.7

H2O NaCl HAc,NaAc

pH7 pH7

3 3 4.58buffer action

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Buffered Solutions

HA + OH- A- + H2O

A- + H3O+ HA + OH-

• Combination of a weak acid and its conjugate base

• Combination of a weak base and its conjugate acid

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Contents



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The Buffer Equation• A Weak Acid and Its Salt

H3O+ + Ac-HAc + H2O

-log[H3O+]= - logKa - log[acid] + log[salt]

salt

acidKa =

[H3O+][Ac-][HAc]

K1[HAc][H2O] = K2[H3O+][Ac-]

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The Buffer Equation

• A Weak Acid and Its Salt

pH= pKa+log [salt][acid]

Buffer equation orHenderson-Hasselbalchequation

Dissociation exponent

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Common ion effect

H3O+ + Ac-HAc + H2O

* when Sod. acetate is added to acetic acid…

Ka =[H3O+][Ac-]

is momentarily disturbed since the acetate ion supplied by the salt increases the [Ac-]

[HAc]

The ionization of HAc is repressed upon the addition of the common ion [Ac-]

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The Buffer Equation• A Weak Base and Its Salt

Kb =[OH-][BH+]

[B]

OH- + BH+B + H2O

salt

base

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The Buffer Equation

• A weak base and its salt

[H3O+] • [OH-] = Kw

[OH-] = Kb[base][salt]

-log[H3O+]= - logKw – log1/Kb - log[salt]/[base]

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The Buffer Equation• A Weak Acid and Its Salt

pH= pKw- pKb + log [base][salt]

* Buffers are not ordinarily prepared from weak bases because of the volatility & instability of the bases and because of the dependence of their pH on pKw, which is often affected by temp. changes.

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Activity coefficientsH3O+ + Ac-HAc + H2O

aH3O+ •aAc-

aHAcKa = [H3O+][Ac-]

[HAc] =

(γH3O+•cH3O+)•(γAc-• CAc-)(γHAc•CHAc)=

Molar conc.

Activity coefficients

Conc. activity

activity

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Activity coefficientsaAc-aHAc

- log[aH3O+] = - log Ka + log

* activity coefficient of the undissociated acid γHAc is essentially 1 and may be dropped.

[salt][acid]pH = pKa + log + log γAc-

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pH 에 영향 주는 인자

1. Altering the ionic strength① Addition of neutral salts② Dilution (alter activity coefficients)

The pH of the most basic buffer was found to change more markedly with temp. than that of acid buffers, owing to Kw.

2. Temperature

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pH indicator• Acid indicator의 경우

HIn + H2O H3O+ + In-

Alkaline colorAcid color

KIn = [H3O+ ][ In-]

[HIn]

base

acid

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PH indicator

• pH = pKIn + log [base][acid]

1/10~10/1

pH =pKIn + 1base acid10/1 1/10

* From experience, one cannot discern a change from the acid color to the salt color the ratio of [base] to [acid] is about 1 to 10

* The effective range of the indicator is…

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pH indicator• Characteristics of colorimetric method

① less accurate② less convenient but less expensive than

electrometric method③ difficult to apply for the unbuffered

pharmaceutical preparation (change the pH -indicator itself is acids or base)④ error may be introduced by the presence of

salts & proteins

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Contents



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Buffer capacity

β=BpH

ΔB : small increment in gram equivalents/Liter of strong(or acid) added to the buffer soln. to produce a pH change of ΔpH

buffer capacity= buffer efficiency

= buffer index= buffer value

• …the magnitude of the resistance of a buffer to pH changes

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Buffer capacity (근사식 이용)

HAc(0.1- 0.01)

NaAC(0.1+ 0.01)0.01

+ NaOH + H2O

pH=pKa + log [salt] + [base][acid] - [base] = 4.85

pH=pKa + log[salt][acid] = 4.76

= 0.010.09

= 0.11=pHβB

• Before the addition of NaOH

• After the addition of NaOH

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• A more exact equation for buffer capacity (1914, 1922)

Buffer capacity

β ---- at any [H3O+].

Ka • [H3O+]β = 2.3 • C •

(Ka + [H3O+])2

c : total buffer conc.(sum of the molar conc. of the acid & the salt)

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• βmax occurs where pH = pKa ([H3O+] = Ka)

Maximum Buffer capacity

( pH = pKa )βmax = 0.576 • C

42.303 • C

[H3O+]2

βmax = 2.303 • C •(2 [H3O+])2

=

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Characteristics of Buffer Capacity

• …is not a fixed value, but rather depend on the amount of base added

• …depends on the value of the ratio [salt]/[acid] and magnitude of the individual concentrations of the buffer components

• The greatest capacity(βmax) occurs where [salt]/[acid] = 1 and pH = pKa

• Because of interionic effects, buffer capacities do not in general exceed a value of 0.2

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• Total buffer capacity of a universal buffer (combination of several buffers)

Universal Buffer

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Contents



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In Vivo biologic buffer systems• Blood① Primary buffers : Plasma ;

NaHCO3-- H2CO3, NaHPO4--NaH2PO4, protein② Secondary buffers : Erythrocytes ;

hemoglobin-oxyhemoglobin, K2Hpo4--KH2PO4

• Lacriminal fluid- pH: 7.4 (range 7 – 8 or slightly higher)

• Urine - pH: 6.0 (range 4.5 – 7.8) - below normal…hydrogen ions are excreted by the kidney.- above pH 7.4…hydrogen ions are retained by action of the kidney.

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Pharmaceutical buffers

• ophthalmic soln.

• colormetric determination of pH

• research studies in which pH must be held constant

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Pharmaceutical buffers• Clark-Lubs mixtures and pH

(a) HCl & KCl, pH 1.2 - 2.2

(b) HCl & potassium biphthalate, pH 2.2 - 4.0

(C) NaOH & potassium biphthalate, pH 4.2 - 5.8

(d) NaOH & KH2PO4 , pH 5.8 - 8.0

(e) H3BO3, NaOH & KCl, pH 8.0 - 10.0

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Preparation of pharmaceutical buffer solutions

• Steps for development of a new buffer① Select a weak acid having a pKa approximately equal

to the pH at which the buffer is to be used.

② Calculate the ratio of salt & weak acid required to obtain the desired pH.

③ Consider the individual conc. Of the buffer salt & acid needed to obtain a suitable buffer capacity

* Individual conc. : 0.05 ~ 0.5M * buffer capacity : 0.01 ~ 0.1

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Preparation of pharmaceutical buffer solutions

• Steps for development of a new buffer

④ Availability of chemicals, sterility of the final soln, stability of the drug & buffer, cost of materials, freedom from toxicity

ex) borate buffer – toxic effect – not be used for oral or parenteral products.

⑤ Determine the pH and buffer capacity using a reliable pH meter

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Buffer in pharmaceutical and biologic systems

• Influence of buffer capacity and pH on tissue irritation

* Tissue irritation will be minimal when…

(a)Buffer solution –β , Volume(b) Physiologic fluid - β , Volume

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• Stability vs. optium therapeutic response

* Undissociated form of a weakly acidic or basic drug has a higher therapeutic activity than the dissociated salt form.

* Molecular form is lipid soluble & can penetrate body membranes readily, where the ionic form, not being lipid soluble, can penetrate membranes only with greater difficulty.

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• pH and solubility* Influence of buffering on the solubility of base

- At a low pH : base is in the ionic form & usually very soluble in aqueous media

- As the pH is raised : more undissociated base is formed when the amount of base exceeds the limited water solubility of this form, free base precipitates from soln.

Base soln. should be buffered at a sufficiently low pH for stabilization against precipitation.

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(Example)

GOAL: Compute the mole percent of free base present on 25℃ and at a pH of 7.4. The pKb of pilocarpineis 7.15 at 25℃.


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• Example

At pH 7.4

C11H16N2O2 + H2O C11H16N2O2H+ + OH-

(Pilocarpine base) (Pilocarpine ion)

pH= pKw- pKb + log [base][salt]

At pH 4.0

7.4 = 14 – 7.15 + log [base][salt]

[base][salt] = 3.56 / 1

4.0 = 14 – 7.15 + log

[base][salt]

Mole percent of base = 3.56 / (1 + 3.56) • 100 =

78%

= 0.0014 / 1

[base][salt]

Mole percent of base = 0.0014 / (1 + 0.0014) • 100 =

0.13%

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Contents



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Buffered isotonic solution

Red blood cell

NaCl solution 2.0 %Hypertonic,

Shrink

0.9 %Isotonic

0.2 %Hypotonic,Hemolysis

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Buffered isotonic solution

• The term Isotonic should be restricted to solutions having equal osmotic pressures which respect to a particular membrane (Husa)

• Isotonicity value…the concentration of an aqueous NaCl soln. having the same colligative properties as soln. (Goyan & Reck)

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Measurement of tonicity• Hemolytic method

…apply red blood cells …based on the fact that a hypotonic soln. liberates oxyhemoglobinin direct proportion to the number of cells hemolyzed

• determine colligative properties (chapter 5)…modifications of the Hill-Blades Technique…based on a measurement of the slight temp. differences arising from differences in the vapor pressure of thermally insulated samples contained in constant-humidity chambers

Tf = 0.52 ºC (Freezing point lowering of human blood & lacrimal fluid)

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Calculating Tonicity Using Liso values

• The Van’t Hoff expression

Tf = L · c(Chapter 6)

Conc. that is isotonic with body fluids

Liso = Tf / c

0.52 °

Molal freezing point depression

of water

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Calculating Tonicity Using Liso values

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Contents



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Method of adjusting tonicity and pH

Class I …add Sod. Chloride to lower the freezing point of soln. to -0.52°

① White-Vincent method② Sprowls method

① Cryoscopic method② Sodium chloride equivalent method

Class II …add Water to form an isotonic soln.

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Class I methods

• Cryoscopic method (빙점강하도법)(Example)How much NaCl is required to render 100mL of a 1% soln. of apomorphine HCl isotonic with blood serum?

Δ Tf0.9% of NaCl soln : 0.52°(Isotonic with blood)Δ Tf1% of apomorphine HCl soln : 0.08° (from table)

to reduce the freezing point by an additional 0.44°(0.52-0.08)Δ Tf1% of NaCl soln : 0.58°

1(%)/X = 0.58/0.44 ; X = 0.76 (%)

Dissolve 1 g apomorphine HCl + 0.76g NaCl make 100mL soln. with water

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Class I methods• Sodium chloride equivalent(E) method

(염화나트륨당량법) by Mellen & Seltzer

1g drug tonicity = Eg NaCl tonicity

ΔTf = Liso · cΔTf = Liso · 1g/MW

c = 1 g / molecular weight

3.4 58.45E

E : weight of NaCl with the same freezing point depression as 1g of the drug.

E ≈ 17 · Liso / MW

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Class II methods

• White-Vincent method

(Example)

GOAL: make 30mL of a 1% soln. of procaine HCl isotonic with body fluid

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Class II methods

• Steps for White-Vincent method① Weight in grams of drug(0.3 g) • Sod. Chloride

equivalent E(0.21..from table) = quantity of sod. Chloride equivalent to w of drug(0.063 g)

② 0.9 g/100mL = 0.063 g / V③ V = 0.063 • 100/0.9④ V = 7.0 mL⑤ Add isotonic-buffered diluting soln. to complete

V = w • E • 111.1

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Class II methods

•White vincent method

0.3g drug(E=0.21) 7ml

add 0.9%NaCl

or

Isotonic buffered sol.

30ml

water

0.9%NaClisotonic

GOAL: make 30mL of 1% soln.

of procaine HClisotonic with body

fluid

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Class II methods

• Sprowls method

w EV

= 0.9 g100 ml

W = 0.3 g (1% solution)

?TABLE

adjusting tonicity

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